Magnetic tape is widely used for recording digital information. One extensive use of digital tape recording is to provide backup and archival storage of vast quantities of digital information, such as records comprising blocks of data. In some applications archival records are recorded on tape in a particular tape format which follows agreed standards at the time the recording was made. The tape may then be placed into archival storage and not retrieved until months or years have passed by. It is not uncommon to specify the useful storage life of recorded digital tapes and cartridges at thirty years, or longer. Whatever may be the useful life of a particular magnetic tapes, a primary assumption on the part of those who store such tapes away is that the recorded information may be read at some date in the future, if access to the archived data is required.
While a particular tape and cartridge may remain functional over many years after being in archival storage, tape transport mechanisms typically do not last nearly so long. Standardized tape recording formats are also susceptible to evolutionary changes and improvements. These changes are primarily driven by improvements in magnetic tape and magnetic head technologies which enable much larger data records and files to be stored on a given area of magnetic tape. One recent development, first employed in the hard disk drive industry, and more recently applied to tape recording, has been the introduction of head assemblies formed of thin film inductive, and magneto-resistive, and giant magneto-resistive (MR) read elements. These elements are typically fabricated in processes including photolithographic patterning steps of the type first developed for use by the semiconductor industry. One desirable aspect of these new thin film MR heads is that head gap widths may be narrowed considerably. Narrower head gaps and finer grain magnetic media coatings on tape mean that many more lineal data tracks may be defined across a magnetic recording tape of a standard given width (such as one-half inch tape). Also, the head structure may be formed as a single small composite structure on a common base or substrate and have as many as 12, or more, distinct heads. By using a common substrate, the heads may be formed to be in a predetermined precise alignment relative to nominal track locations defined along the magnetic tape. With e.g. 12 write and read head elements of the head structure in precise alignment with the defined nominal track locations, and with large scale integrated chips providing multiple data write/read channels, it has now become practical to have e.g. 12 channels for simultaneously writing user data to tape and for reading user data back from tape. This increase in the number of write/read channels effectively increases the overall data transfer rate between a host computer and the tape drive, and enables the tape drive to be characterized as having higher performance than previously available.
In order to take full advantage of the new thin film MR head technology in tape drives, a track layout which differs from previous standard track formats is required. This new track layout employs tracks of much narrower track width and pitch. Since the write/read heads are grouped together on a common fabrication substrate, the data tracks are also grouped together. In one arrangement, the data tracks are grouped into bands, or zones, across the tape, such that e.g. ten lateral head positions relative to the tape within a single zone would access 120 tracks. When a zone boundary is reached, the head structure or assembly is then displaced laterally relative to the tape travel path to the next zone, and the tracks of that zone then become accessible. Because track widths are very narrow, enabling track densities of e.g. 2000 tracks per inch, or higher, lateral tape motion must be followed in order to keep the new head assemblies in alignment with the tracks during tape travel past the head. Magnetic servo patterns written onto the tape may be read by servo readers and used to generate position error signals used by a closed loop positioner to correct head position. Alternatively, optical servo patterns embossed or otherwise formed on a back side of the tape may be used to provide position error signals, as disclosed for example in commonly assigned, co-pending U.S. patent application Ser. No. 09/046,723 filed on Mar. 24, 1998, and entitled: "Multi-Channel Magnetic Tape System Having Optical Tracking Servo", the disclosure thereof being incorporated herein by reference.
The later high-density track format differs from previous standard formats. For example, FIG. 1 shows an existing standard tape format employing longitudinal recording. In this example a magnetic recording tape 10 has a series of parallel longitudinal tracks. Three tracks 12A, 12B and 12C are shown in the FIG. 1 example, although more tracks, such as 24, 48, 96 or 128 tracks may be employed in a one-half inch tape lineal format in accordance with a particular standardized track layout plan. A head assembly 14 includes e.g. discrete inductive read or write head elements 14A, 14B and 14C which are aligned with the tracks 12A, 12B and 12C. Other tracks may be accessed by displacing the head assembly 14 laterally relative to the direction of the tape along a path indicated by the vertical arrows axial aligned with the head 14 in the FIG. 1 view.
Another preexisting standard tape format employs azimuth recording of the data tracks, i.e. adjacent tracks are recorded with magnetic gaps oblique to each other, creating what appears generally as a "herringbone" pattern, shown in FIG. 2. Therein, one track 16A has its magnetic flux reversal pattern aligned with a first azimuth angle oblique to the tape travel direction, and an adjacent track 16B has its magnetic flux reversal pattern aligned with a second azimuth angle in an opposite sense of the first angle relative to a travel path of the magnetic tape 10. One known advantage derived from azimuth recording is that lineal guard bands or regions between tracks may be reduced, and the tracks may be placed closer together and read back without interference from adjacent tracks. While azimuth recording technology increases track density somewhat, complications arise in writing and reading the slanted tracks. Multi-element tape heads, such as the tape head 100 shown in FIGS. 4-6 of U.S. Pat. No. 5,452,152, can be provided with some of the write/read elements having magnetic gaps aligned with one azimuth angle, and other write/read elements having magnetic gaps aligned with the other azimuth angle. Such heads are then positioned laterally relative to the direction of tape travel in order to come into alignment with particular tracks. An alternative approach, also shown in FIG. 2 and enabling compatibility with both the longitudinal tracks 12A, 12B and 12C of the FIG. 1 example, and the azimuth tracks 16A and 16B of the FIG. 2 example, calls for rotating a head 19 having perpendicular head elements 19A and 19B between the two azimuth formats and the longitudinal format. One example of a multi-element head is given in commonly assigned, co-pending U.S. patent application Ser. No. 08/760,794, now U.S. Pat. No. 6,867,339, filed on Dec. 4, 1996, and entitled: "Four Channel Azimuth and Two Channel Non-Azimuth Read-After-Write Longitudinal Magnetic Head", the disclosure thereof being incorporated herein by reference. An example of an azimuth tape recording pattern and an apparatus for writing the pattern in accordance with servo information read back from an adjacent track is given in commonly assigned U.S. Pat. No. 5,371,638, the disclosure thereof being incorporated by reference.
FIG. 3 illustrates a newer track format plan employing a tape 10A carrying high recording density magnetic media. According to the FIG. 3 track plan, a multiplicity of data tracks 20n are distributed across e.g. five zones 22A, 22B, 22C, 22D and 22E. A monolithic thin film head element 24 within the head assembly includes e.g. 12 write-read elements in relatively close proximity enabling writing to and reading from tracks of a particular zone, e.g. zone 22D in the FIG. 3 example. Other zones may be accessed by displacing the head assembly laterally relative to the direction of travel of tape 10A. Further details of a tape and tape drive in accordance with this general approach may be found in the above-referenced U.S. patent application Ser. No. 09/046,723.
While the standardized longitudinal recording patterns shown in the FIG. 1 example, and the azimuth recording patterns shown in the FIG. 2 example, have worked very well for a number of years, newer higher density track layout patterns and plans, enabled by multi-element thin film head as well as improvements in tape media technologies are now proposed and will most likely become standard approaches in the future for certain categories of longitudinal digital tape recording methods and devices. Since extensive cartridge handling equipment in use is capable of handling standard cartridges containing tape having the newer format, no compelling need has arisen to change the cartridge form factor or major features in order to accommodate the new tape track formats enabled by emerging new technologies. Yet, a hitherto unsolved need has remained for backward compatibility within tape drive units having monolithic multi-element heads by enabling reading back of older preexisting tape formats recorded on tape carried in standard tape cartridges, but based on discrete head elements, in order to recover archival data recorded on the older tapes.